valgrind doesn't recognize invalid write - c++

Question

Valgring doesn`t detect memory errors.
I am using valgrind 3.11, gcc 5.4.0 under ubuntu and
have an incorrect code in my program, like in the sample.
I analyzed this program, using valgrind. But valgrind doesn't report about any errors.
#include <string.h>
int main(){
int a[3];
memcpy(a,"aaabbbcccdddeeefffggghhh", 24);
return 0;
}
What's wrong with valgrind?

Answer 1

valgrind doesn't know a, not its size then, while you stay in the stack it cannot detect the error
To compare, having that :
#include <string.h>
int main(){
int * a = new int[3];
memcpy(a,"aaabbbcccdddeeefffggghhh", 24);
return 0;
}
valgrind can detect the error because it knows the size of the allocated block :
pi#raspberrypi:/tmp $ valgrind ./a.out
==16164== Memcheck, a memory error detector
==16164== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==16164== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16164== Command: ./a.out
==16164==
==16164== Invalid write of size 8
==16164== at 0x4865F44: ??? (in /usr/lib/arm-linux-gnueabihf/libarmmem.so)
==16164== Address 0x4bc9f60 is 8 bytes inside a block of size 12 alloc'd
==16164== at 0x48485F0: operator new[](unsigned int) (vg_replace_malloc.c:417)
==16164== by 0x105A7: main (v.cc:3)
==16164==
==16164== Invalid write of size 8
==16164== at 0x4865F54: ??? (in /usr/lib/arm-linux-gnueabihf/libarmmem.so)
==16164== Address 0x4bc9f68 is 4 bytes after a block of size 12 alloc'd
==16164== at 0x48485F0: operator new[](unsigned int) (vg_replace_malloc.c:417)
==16164== by 0x105A7: main (v.cc:3)
==16164==
==16164==
==16164== HEAP SUMMARY:
==16164== in use at exit: 12 bytes in 1 blocks
==16164== total heap usage: 2 allocs, 1 frees, 20,236 bytes allocated
==16164==
==16164== LEAK SUMMARY:
==16164== definitely lost: 12 bytes in 1 blocks
==16164== indirectly lost: 0 bytes in 0 blocks
==16164== possibly lost: 0 bytes in 0 blocks
==16164== still reachable: 0 bytes in 0 blocks
==16164== suppressed: 0 bytes in 0 blocks
==16164== Rerun with --leak-check=full to see details of leaked memory
==16164==
==16164== For counts of detected and suppressed errors, rerun with: -v
==16164== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 3)